This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatograhic printing process involves the recordation of electrostatic latent charge patterns and the reproduction thereof in viewable form. The field of electrostatography includes electrophotography and electrography. Electrophotography is that class of electrostatography which employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent charge pattern. Electrography is that class of electrostatography which utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic charge pattern. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of these processes.
In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level and exposed to a light image of an original document being reproduced. The light image discharges the photoconductive member in the irradiated areas to record an electrostatic latent image thereon. Thereafter, charged particles are attracted to the latent image so as to form a powder image on the photoconductive surface. The powder image is then transferred to a sheet of support material, i.e. the copy sheet, in image configuration. The powder image adhering to the copy paper is heated permanently affixing it thereto. This process is more fully described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, and further exemplified by many other related patents in the art.
The act of depositing charged particles on the electrostatic latent image is termed development. Development is employed in all of the aforementioned classes of electrostatography. In electrophotographic printing, the electrostatic latent image recorded on the photoconductive member is brought into contact with a developer mix. Typically, the developer mix comprises dyed or colored thermoplastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferro-magnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. When the developer mix is brought into contact with the charged photoconductive surface, the greater attractive force of the electrostatic latent image recorded thereon causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image.
Recently, single component developer materials have been developed. These are carrierless magnetic particles which are attracted from the developed unit to the electrostatic latent image recorded on the photoconductive surface. U.S. Pat. No. 3,909,258 issued to Kotz in 1975 discloses a development system employing single component magnetic toner particles. Similarly, U.S. Pat. No. 2,846,333 issued to Wilson in 1958 describes a magnetic brush developer unit which utilizes single component magnetic particles. Finally, U.S. Pat. No. 3,563,734 issued to Shely in 1971 discloses the utilization of conductive magnetic particles employed in a magnetic brush developer unit for rendering visible a charge pattern recorded on a surface. However, systems of this type are not maintenance free.
In order to design a system which would be substantially free of maintenance, the magnetic member must be completely isolated from the toner laden environment in which it must function. More particularly, the bearings on supporting the rotating member must be isolated from the particles developing the latent image. Standard techniques for sealing the bearings are generally inappropriate inasmuch as they generally permit small particles, i.e. about 2 microns in size, to contaminate the bearings. Alternatively, if sufficient grease is packed into the bearings to exclude foreign material therefrom, the grease may seep from the seal contaminating the particles and causing other system problems. Tight fitting seals are another option, however, they significantly increase the torque and wear of the system. Thus, seals of this type are not highly desirable. An additional requirement of this type of development system is that the gap between the donor roll supporting the particles and the photoconductive member must remain substantially constant. One solution to this problem has been to have the particle support donor roll ride on the photoconductive member. This imposes another constraint on the system. The drive system must move or be coupled flexible to the supporting donor roll when the supporting donor roll moves laterally to track the out of round condition of the photoconductive member.
Accordingly, it is a primary object of the present invention to improve the donor roll employed in the development system of an electrophotographic printing machine by eliminating external moving parts.